An MOS transistor includes a substrate, a first region, a second region, a source region, a drain region, an active gate stack, and a dummy gate stack. The substrate has a first conductivity. The first region having the first conductivity is formed in the substrate. The second region having a second conductivity is formed in the substrate and is adjacent to the first region. The source region with the second conductivity is formed in the first region. The drain region with the second conductivity is formed in the second region. The active gate stack is disposed on the first region. The dummy gate stack is disposed on the second region, and the dummy gate stack is electrically coupled to a variable voltage. Embodiments relate to an LDMOS transistor.